The invention relates to improvements in methods of and in apparatus for drying elongated substrates in the form of webs or strips which are made of paper, textile material or the like. More particularly, the invention relates to improvements in methods of and in apparatus for expelling volatile constituents (particularly solvents) from coatings of elongated substrates.
It is already known to expel solvents and/or other volatile constituents (hereinafter called solvents) from the coatings of running substrates by contacting the substrates with an inert gas, for example, nitrogen. Such treatment (drying) of coated substrates is desirable and advantageous if it involves expulsion of solvents from coatings of highly sensitive substrates or from highly sensitive coatings of substrates. Examples of such substrates are magnetic tapes and webs which are coated with adhesive substances. Convective drying has been found to be particularly effective in connection with the expulsion of solvents from such coatings. Proper selection of parameters (such as pressure, temperature and velocity) of inert gases which are used to expel volatile constituents from coated substrates in the form of magnetic tapes is particularly important because such parameters determine the surface roughness of the finished product. As a rule, the inert gas is nitrogen which should contain less than 1% oxygen in order to reduce the danger of explosion as a result of saturation of inert gas with solvents in the course of the drying operation. Presently preferred driers are so-called nozzle driers, and their exact design depends on the sensitivity of the substrate and/or the coating which is carried by the substrate. For example, if the substrate and its coating constitute a lightweight magnetic tape, the drier is preferably a suspension drier which is designed to maintain the running substrate in a suspended state, i.e., the running substrate is completely surrounded by a stream of inert gas which is discharged by suitably positioned and oriented nozzles. If the substrate and its coating constitute a relatively heavy web, the drier can be provided (a) with rollers or pulleys which contact the uncoated side of the substrate and (b) with nozzles which direct jets of inert gas against the coating at the other side of the substrate.
A presently known apparatus for expelling solvents from coatings on substrates is disclosed in commonly owned U.S. Pat. No. 4,886,564 granted Dec. 12, 1989 to Ralf Pagendarm et al. for "Method of and apparatus for applying coating material to a running substrate".